I was also puzzled by Rashi, since the Gemara clearly states that Tzarda is permitted and Barda is forbidden, and they are not interchangeable. I suppose that Rashi should really read "Tzarda Shari" and then "Barda," or some other similar form of correction.

Yoel Domb

In earlier printings of the Gemara, which do not make a clear distinction between Rashi's Dibur ha'Maschil and his Perush (such as our modern editions do, with the DH in bold print, etc.), Rashi simply lists these two words, without any commentary. When the Gemara discusses new and rare words, especially ones that are similar to each other, Rashi makes a list of those words so that one should not make a mistake in the Girsa. (Rashi also does this two lines later, where he just lists the words Chova, Chuga, Harnuga, Suga, etc.)

Moreover, in the case of Tzarda and Barda, Rashi lists the two names to emphasize why the Gemara finds a need to make a Siman in order to remember the Din. This is because the names of these birds are similar and it is easy to get confused.

Y. Shaw